Photonic crystal waveguide and directional coupler using the same

ABSTRACT

The present invention provides a photonic crystal waveguide comprising: a substrate; a bottom cladding layer over the substrate; and a core layer over the bottom cladding layer, the core layer having a uniform distribution of holes, wherein the core layer has at least a waveguide region which is thicker than a remaining region of the core layer to cause a refractive index guide effect, or wherein the core layer has at least a waveguide region, on which a dielectric pattern is provided which has a refractive index higher than a substance in contact with a top surface of the core layer, or wherein the core layer has at least a waveguide region, and the holes except on the waveguide region are filled with an air, whilst the holes on the waveguide regions are filled with a filler material having a refractive index higher than 1.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a photonic crystal waveguide,and a directional coupler using the same and more particularly to aphotonic crystal waveguide in a form of a photonic crystal micro opticalcircuit for optical communications.

[0002] It has been known in the art that the photonic crystal waveguideis used as an optical filter for selecting transmittable opticalwavelength, an optical multiplexer/demultiplexer, and an opticaldispersion-compensating device in the various fields of opticalcommunication systems, optical switching systems and optical measuringsystems. The directional coupler uses the photonic crystal waveguide.

[0003] In recent years, a photonic crystal has received a great deal ofattention as a three-dimensional periodic structure having a refractiveindex in the same order as the optical wavelength. This photonic crystalhas a potential capability of a remarkable size reduction of the opticalcircuit by three digits or more. For this reason, it has been on thegreat expectation to apply the photonic crystal to the micro-opticalcircuit. Various structures of the optical waveguide in the form of themicro-optical circuit formed in the photonic crystal have been proposed.

[0004]FIG. 1 is a fragmentary schematic perspective view illustrative ofa first conventional optical waveguide in the photonic crystal. Theoptical waveguide comprises a line defect introduced in the photoniccrystal which has a complete photonic band gap to a wavelength of theoptical wave to be propagated through the optical waveguide. Namely, theline defect in the photonic crystal is used as the optical waveguide.This line defect optical waveguide has a high optical confinementfunction, for which reason the line defect optical waveguide isresponsible to a abrupt or tight curve. Thus, the line defect opticalwaveguide provides a large freedom in pattern of the optical circuit andalso allows a remarkable size reduction of the optical circuit, Thephotonic crystal has a three dimensional periodical structure whichcomprises lamination structures of a bottom cladding layer 11 of a firstmaterial having a low refractive index, a core layer 12 of a secondmaterial having a high refractive index and a top cladding layer 13 ofthe first material, wherein the core layer 12 is sandwiched between thetop and bottom cladding layers 11 and 13. The core layer 12 has a highrefractive index, whilst the top and bottom cladding layers 11 and 13have a low refractive index. The core layer 12 may be made of silicon.Each of the top and bottom cladding layers 11 and 13 may be made ofsilicon dioxide. The three dimensional periodical structure has aphotonic band gap defined by forbidden bands against the propagation ofa light having a specific wavelength. If a light is generated in thephotonic crystal having the photonic band gap, then the light isconfined in the photonic crystal, wherein the propagation of the lightis inhibited. The complete photonic band gap inhibits the threedimensional propagation of light. If the line defect is introduced intothe photonic crystal having the complete photonic band gap, the linedefect permits the propagation of light along the line defect in thephotonic crystal. The line defect serves as the waveguide in thephotonic crystal. In FIG. 1, hexagons represent lattice structures ofthe crystal. The photonic crystal has lattice defects aligned in adirection along arrow marks, wherein the lattice defects are representedby the absences of the hexagons. The incident light 10 represented bythe arrow mark is propagated through the line defect in the photoniccrystal.

[0005] Japanese laid-open patent publication No. 11-218627 discloses thefollowing second conventional technique for stabilizing properties ofthe photonic crystal waveguide and reducing the manufacturing cost. Thesecond conventional photonic crystal waveguide has a dielectric slabwaveguide over a surface of a silicon substrate, wherein the dielectricslab waveguide has a matrix array of a lattice array of refractive indexvarying regions which are different in refractive index from a corelayer of the dielectric slab waveguide. The refractive index varyingregions are made of the same material as the core layer, and have beensubjected to a refractive index varying treatment due to an opticalinduced effect. The dielectric slab waveguide comprises laminations of abottom cladding layer, the core layer and a top cladding layer. Thoselamination structure may be formed over the substrate by a metal organicchemical vapor deposition method or a liquid phase epitaxy to completethe slab waveguide over the substrate. Subsequently, the core layer issubjected to a selective irradiation through the top cladding layer withany one of an electron beam, a synchrotron orbital radiation light, aultraviolet ray, and an infrared ray in order to cause a variation inrefractive index, due to the optical induced effect, of the irradiatedparts of the core layer of the slab waveguide, whereby the refractiveindex varying regions are formed, which are different in refractiveindex from the remaining non-irradiated parts of a core layer of thedielectric slab waveguide.

[0006] The above first and second conventional photonic crystalwaveguides have the following disadvantages. A sectioned area of theline defect waveguide is extremely small and it is difficult to obtain asufficient optical coupling with any external optical system. Theactually available method of forming the line defect waveguide has notyet been established.

[0007] In the above circumstances, it had been required to develop anovel photonic crystal waveguide free from the above problem.

SUMMARY OF THE INVENTION

[0008] Accordingly, it is an object of the present invention to providea novel photonic crystal waveguide free from the above problems.

[0009] It is a further object of the present invention to provide anovel photonic crystal waveguide having a high optical couplingcoefficient in coupling the photonic crystal waveguide to an externaloptical system.

[0010] It is a still further object of the present invention to providea novel photonic crystal waveguide which is suitable for manufacturingthe same.

[0011] It is yet a further object of the present invention to provide anovel directional coupler utilizing the novel photonic crystal waveguidefree from the above problems.

[0012] It is a further object of the present invention to provide anovel directional coupler utilizing the novel photonic crystal waveguidehaving a high optical coupling coefficient in coupling the photoniccrystal waveguide to an external optical system.

[0013] It is a still further object of the present invention to providea novel directional coupler utilizing the novel photonic crystalwaveguide which is suitable for manufacturing the same.

[0014] It is yet a further object of the present invention to provide anovel directional coupler utilizing the novel directional couplerutilizing the novel photonic crystal waveguide.

[0015] It is another object of the present invention to provide a novelmethod of use of a novel photonic crystal waveguide free from the aboveproblems.

[0016] It is a further object of the present invention to provide anovel method of use of a novel photonic crystal waveguide having a highoptical coupling coefficient in coupling the photonic crystal waveguideto an external optical system.

[0017] It is a still further object of the present invention to providea novel method of use of a novel photonic crystal waveguide which issuitable for manufacturing the same.

[0018] It is another object of the present invention to provide a novelmethod of forming a novel photonic crystal waveguide free from the aboveproblems.

[0019] It is a further object of the present invention to provide anovel method of forming a novel photonic crystal waveguide having a highoptical coupling coefficient in coupling the photonic crystal waveguideto an external optical system.

[0020] It is a still further object of the present invention to providea novel method of forming a novel photonic crystal waveguide which issuitable for manufacturing the same.

[0021] The first present invention provides a photonic crystal waveguidecomprising: a substrate; a bottom cladding layer over the substrate; anda core layer over the bottom cladding layer, the core layer having auniform distribution of holes, wherein the core layer has at least awaveguide region which is thicker than a remaining region of the corelayer to cause a refractive index guide effect.

[0022] The second present invention provides a photonic crystalwaveguide comprising: a substrate; a bottom cladding layer over thesubstrate; and a core layer with a uniform thickness over the bottomcladding layer, the core layer having a uniform distribution of holes;wherein the core layer has at least a waveguide region, on which adielectric pattern is provided which has a refractive index higher thana substance in contact with a top surface of the core layer.

[0023] The third present invention provides a photonic crystal waveguidecomprising: a substrate; a bottom cladding layer over the substrate; anda core layer with a uniform thickness over the bottom cladding layer,the core layer having a uniform distribution of holes wherein the corelayer has at least a waveguide region, and the holes except on thewaveguide region are filled with an air, whilst the holes on thewaveguide regions are filled with a filler material having a refractiveindex higher than 1.

[0024] The above and other objects, features and advantages of thepresent invention will be apparent from the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Preferred embodiments according to the present invention win bedescribed in detail with reference to the accompanying drawings.

[0026]FIG. 1 is.

[0027]FIG. 2 is a fragmentary perspective view illustrative of a firstnovel photonic crystal waveguide in a first embodiment in accordancewith the present invention.

[0028]FIG. 3 is a fragmentary perspective view illustrative of a secondnovel photonic crystal waveguide in a second embodiment in accordancewith the present invention.

[0029]FIG. 4 is a fragmentary perspective view illustrative of a thirdnovel photonic crystal waveguide in a third embodiment in accordancewith the present invention.

[0030]FIGS. 5A through 5C are fragmentary perspective views illustrativeof the third novel photonic crystal waveguides in sequential stepsinvolved in the fabrication method therefor in the third embodiment inaccordance with the present invention.

[0031]FIG. 6 is a fragmentary perspective view illustrative of a fourthnovel photonic crystal waveguide in a fourth embodiment in accordancewith the present invention.

[0032]FIGS. 7A through 7C are fragmentary perspective views illustrativeof the fourth novel photonic crystal waveguides in sequential stepsinvolved in the fabrication method therefor in the fourth embodiment inaccordance with the present invention.

[0033]FIG. 8 is a fragmentary perspective view illustrative of a fifthnovel photonic crystal waveguide to be used as a directional coupler ina fifth embodiment in accordance with the present invention.

[0034]FIG. 9 is a fragmentary perspective view illustrative of the fifthnovel photonic crystal waveguide of FIG. 8 in use,

[0035]FIG. 10 is a fragmentary perspective view illustrative of a sixthnovel photonic crystal waveguide in a sixth embodiment in accordancewith the present invention.

[0036]FIG. 11 is a fragmentary perspective view illustrative of aseventh novel photonic crystal waveguide in this sixth embodiment inaccordance with the present invention.

[0037]FIGS. 12A and 12B are fragmentary perspective views illustrativeof a novel method of forming the photonic crystal waveguide in thissixth embodiment in accordance with the present invention.

[0038]FIG. 13 is a fragmentary perspective view illustrative of aneighth novel photonic crystal waveguide in this sixth embodiment inaccordance with the present invention.

DISCLOSURE OF THE INVENTION

[0039] The first present invention provides a photonic crystal waveguidecomprising: a substrate; a bottom cladding layer over the substrate; anda core layer over the bottom cladding layer, the core layer having auniform distribution of holes, wherein the core layer has at least awaveguide region which is thicker than a remaining region of the corelayer to cause a refractive index guide effect.

[0040] It is preferable that the waveguide region has a ridged shape.

[0041] It is also preferable that a plurality of the waveguide regionextends in parallel to each other and distanced from each other to forma directional copular.

[0042] It is also preferable that the core layer is made of such aphotonic crystal material that a wavelength of a light to be propagatedthrough the waveguide region is in the vicinity of a photonic band gapedge of the photonic crystal material in order to utilize an intensedispersion phenomenon.

[0043] It is also preferable that the uniform distribution of the holescomprises a two-dimensional periodical array of through holes at aconstant pitch between centers of adjacent two of the through holes.

[0044] It is also preferable that the uniform distribution of the holescomprises a three-dimensional periodical array of holes at a constantpitch between centers of adjacent two of the through holes.

[0045] It is also preferable that the holes are filled with an air.

[0046] It is also preferable to further comprise a top cladding layerover the core layer.

[0047] It is also preferable that a dielectric pattern is provided onthe waveguide region, and the dielectric pattern has a refractive indexhigher than a substance in contact with a top surface of the core layer.

[0048] It is also preferable that the holes except on the waveguideregion are filled with an air, whilst the holes on the waveguide regionsare filled with a filler material having a refractive index higher than1.

[0049] It is also preferable that a dielectric pattern is provided onthe waveguide region, and the dielectric pattern has a refractive indexhigher than a substance in contact with a top surface of the core layer,and that the holes except on the waveguide region are filled with anair, whilst the holes on the waveguide regions are filled with a fillermaterial having a refractive index higher than 1.

[0050] The second present invention provides a photonic crystalwaveguide comprising: a substrate; a bottom cladding layer over thesubstrate; and a core layer with a uniform thickness over the bottomcladding layer, the core layer having a uniform distribution of holes;wherein the core layer has at least a waveguide region, on which adielectric pattern is provided which has a refractive index higher thana substance in contact with a top surface of the core layer.

[0051] It is preferable that the substance in contact with the topsurface of the core layer is an air, and the refractive index of thedielectric pattern is higher than 1.

[0052] It is also preferable to further comprise a top cladding layerover the core layer, and the top cladding layer is made of the samematerial as the bottom cladding layer, and the substance in contact withthe top surface of the core layer is the same material as the bottomcladding layer, and the refractive index of the dielectric pattern ishigher than the top cladding layer.

[0053] It is also preferable that a plurality of the waveguide regionextends in parallel to each other and distanced from each other to forma directional copular.

[0054] It is also preferable that the core layer is made of such aphotonic crystal material that a wavelength of a light to be propagatedthrough the waveguide region is in the vicinity of a photonic band gapedge of the photonic crystal material in order to utilize an intensedispersion phenomenon.

[0055] It is also preferable that the uniform distribution of the holescomprises a two-dimensional periodical array of through holes at aconstant pitch between centers of adjacent two of the through holes.

[0056] It is also preferable that the uniform distribution of the holescomprises a three-dimensional periodical array of holes at a constantpitch between centers of adjacent two of the through holes.

[0057] It is also preferable that the holes are filled with an air.

[0058] It is also preferable that the waveguide region is thicker than aremaining region of the core layer to cause a refractive index guideeffect.

[0059] It is also preferable that the holes except on the waveguideregion are filled with an air, whilst the holes on the waveguide regionsare filled with a filler material having a refractive index higher than1.

[0060] It is also preferable that the waveguide region is thicker than aremaining region of the core layer to cause a refractive index guideeffect, and that the holes except on the waveguide region are filledwith an air, whilst the holes on the waveguide regions are filled with afiller material having a refractive index higher than 1.

[0061] The third present invention provides a photonic crystal waveguidecomprising: a substrate; a bottom cladding layer over the substrate; anda core layer with a uniform thickness over the bottom cladding layer,the core layer having a uniform distribution of holes wherein the corelayer has at least a waveguide region, and the holes except on thewaveguide region are filled with an air, whilst the holes on thewaveguide regions are filled with a filler material having a refractiveindex higher than 1.

[0062] It is also preferable that a plurality of the waveguide regionextends in parallel to each other and distanced from each other to forma directional copular.

[0063] It is also preferable that the core layer is made of such aphotonic crystal material that a wavelength of a light to be propagatedthrough the waveguide region is in the vicinity of a photonic band gapedge of the photonic crystal material in order to utilize an intensedispersion phenomenon.

[0064] It is also preferable that the uniform distribution of the holescomprises a two-dimensional periodical array of through holes at aconstant pitch between centers of adjacent two of the through holes.

[0065] It is also preferable that the uniform distribution of the holescomprises a three-dimensional periodical array of holes at a constantpitch between centers of adjacent two of the through holes.

[0066] It is also preferable to further comprise a top cladding layerover the core layer.

[0067] It is also preferable that the filler material has a temperaturecoefficient which is inverse in sign to a temperature coefficient of abase material of the core layer.

[0068] It is also preferable that the waveguide region is thicker than aremaining region of the core layer to cause a refractive index guideeffect.

[0069] It is also preferable that a dielectric pattern is provided onthe waveguide region, and the dielectric pattern has a refractive indexhigher than a substance in contact with a top surface of the core layer.

[0070] It is also preferable that the waveguide region is thicker than aremaining region of the core layer to cause a refractive index guideeffect, and that a dielectric pattern is provided on the waveguideregion, and the dielectric pattern has a refractive index higher than asubstance in contact with a top surface of the core layer.

[0071] The fourth present invention provides a directional couplercomprising a substrate; a bottom cladding layer over the substrate; anda photonic crystal core layer over the bottom cladding layer, the corelayer having a uniform distribution of holes, and the core layer beingmade of such a photonic crystal material that a wavelength of a light tobe propagated through the waveguide region is in the vicinity of aphotonic band gap edge of the photonic crystal material in order toutilize an intense dispersion phenomenon; and the core layer having apair of stripe-shaped waveguide regions which extends in parallel toeach other, wherein the stripe-shaped waveguide regions are thicker thana remaining region of the core layer to cause a refractive index guideeffect.

[0072] It is also preferable that the uniform distribution of the holescomprises a two-dimensional periodical array of through holes at aconstant pitch between centers of adjacent two of the through holes.

[0073] It is also preferable that the uniform distribution of the holescomprises a three-dimensional periodical array of holes at a constantpitch between centers of adjacent two of the through holes.

[0074] It is also preferable to further comprise a top cladding layerover the core layer.

[0075] It is also preferable that the waveguide region has a ridgedshape.

[0076] It is also preferable that the holes are filled with an air.

[0077] The fifth present invention provides a directional couplercomprising: a substrate; a bottom cladding layer over the substrate; anda photonic crystal core layer over the bottom cladding layer, the corelayer having a uniform distribution of holes, and the core layer beingmade of such a photonic crystal material that a wavelength of a light tobe propagated through the waveguide region is in the vicinity of aphotonic band gap edge of the photonic crystal material in order toutilize an intense dispersion phenomenon and the core layer having apair of stripe-shaped waveguide regions which extends in parallel toeach other, wherein dielectric patterns is provided on the waveguideregions, and the dielectric patterns have a refractive index higher thana substance in contact with a top surface of the core layer.

[0078] It is also preferable that the substance in contact with the topsurface of the core layer is an air, and the refractive index of thedielectric pattern is higher than 1.

[0079] It is also preferable to further comprise a top cladding layerover the core layer, and the top cladding layer is made of the samematerial as the bottom cladding layer, and the substance in contact withthe top surface of the core layer is the same material as the bottomcladding layer, and the refractive index of the dielectric pattern ishigher than the top cladding layer.

[0080] It is also preferable that the uniform distribution of the holescomprises a two-dimensional periodical array of through holes at aconstant pitch between centers of adjacent two of the through holes.

[0081] It is also preferable that the uniform distribution of the holescomprises a three-dimensional periodical array of holes at a constantpitch between centers of adjacent two of the through holes.

[0082] It is also preferable that the holes are filled with an air.

[0083] The sixth present invention provides a directional couplercomprising: a substrate; a bottom cladding layer over the substrate; anda photonic crystal core layer over the bottom cladding layer, the corelayer having a uniform distribution of holes, and the core layer beingmade of such a photonic crystal material that a wavelength of a light tobe propagated through the waveguide region is in the vicinity of aphotonic band gap edge of the photonic crystal material in order toutilize an intense dispersion phenomenon; and the core layer having apair of stripe-shaped waveguide regions which extends in parallel toeach other, wherein the core layer has at least a waveguide region, andthe holes except on the waveguide region are filled with an air, whilstthe holes on the waveguide regions are filled with a filler materialhaving a refractive index higher than 1.

[0084] It is also preferable that the uniform distribution of the holescomprises a two-dimensional periodical array of through holes at aconstant pitch between centers of adjacent two of the through holes.

[0085] It is also preferable that the uniform distribution of the holescomprises a three-dimensional periodical array of holes at a constantpitch between centers of adjacent two of the through holes.

[0086] It is also preferable to further comprise a top cladding layerover the core layer.

[0087] It is also preferable that the filler material has a temperaturecoefficient which is inverse in sign to a temperature coefficient of abase material of the core layer.

PREFERRED EMBODIMENT

[0088] First Embodiment:

[0089] A first embodiment according to the present invention will bedescribed in detail with reference to the drawings. FIG. 2 is afragmentary perspective view illustrative of a first novel photoniccrystal waveguide in a first embodiment in accordance with the presentinvention. The first novel photonic crystal waveguide has atwo-dimensional slab photonic crystal layered structure. Thistwo-dimensional slab photonic crystal layered structure comprises abottom cladding layer 2 over a silicon substrate 1 and a photoniccrystal core layer 3 over the bottom cladding layer 2. The bottomcladding layer 2 is made of silicon dioxide having a refractive index ofabout 1.5. A top surface of the photonic crystal core layer 3 is exposedto an air. This air serves as a top cladding layer having a refractiveindex of 1. The silicon dioxide bottom cladding layer 2 may be laminatedover the silicon substrate 1 by a self-cloning method which is disclosedin 1997, electronics letters, vol. 33, p. 1260, entitled “fabrication ofsub-micrometer 3D periodic structures composed of Si/SiO₂”. The photoniccrystal core layer 3 has arrays of through holes 30 which completelypenetrate the photonic crystal core layer 3 and reach the top surface ofthe bottom cladding layer 2, wherein the through holes 30 aredistributed entirely throughout the photonic crystal core layer 3 at acenter pitch in the range of 0.6-0.8 micrometers. The diameter of thethrough holes 30 may be about the wavelength of the propagating light,The center pitch is defined to be a distance between centers of adjacenttwo of the distributed through holes 30. The through holes 30 have adiameter of about 0.5 micrometers. The distribution of the through holes30 may comprise a two-dimensional periodical array, for example, in theform of square-lattice, triangle-lattice or hexagonal-lattice.

[0090] In this case, an averaged refractive index of the photoniccrystal layer is approximately 2. In view of both the opticalconfinement and the coupling coefficient with the external opticalsystem, it is preferable that the silicon dioxide bottom cladding layer2 and the photonic crystal core layer 3 have a thickness of about 2micrometers. A resist is applied on the top surface of the photoniccrystal core layer 3. The resist is then patterned by lithographyprocesses to form a stripe-shaped resist pattern 4 on a waveguide regionof the photonic crystal core layer 3. In the slab photonic crystalwaveguide, the waveguide region covered by the stripe-shaped resistpattern 4 is higher in equivalent refractive index than the remainingpart of the photonic crystal core layer 3. A light is confined in theregion which is higher in equivalent refractive index than thesurrounding region of the photonic crystal core layer 3, for whichreason the light is confined in the waveguide region which is higher inequivalent refractive index than the remaining part of the photoniccrystal core layer 3. Accordingly, the waveguide region higher inequivalent refractive index serves as an optical waveguide. The width ofthe stripe-shaped resist pattern 4 defines a width of the opticalwaveguide. In view of both the optical confinement and the couplingcoefficient with the external optical system, it is preferable that thewidth of the stripe-shaped resist pattern 4 is about 2 micrometers, sothat the width of the optical waveguide is about 2 micrometers. Asdescribed above, the preferable thickness of the photonic crystal corelayer 3 is also 2 micrometers. Thus, the optical waveguide has asquare-sectioned area of 2 micrometers×2 micrometers, which is muchlarger than the above conventional line defect optical waveguide. Thislarge sectioned area of the optical waveguide is suitable for obtaininga high optical coupling coefficient to the external optical system.

[0091] In accordance with the first novel two-dimensionalslab-structured photonic crystal waveguide, in place of the conventionalline defect optical waveguide, the resist pattern 4 is provided fordefining the optical waveguide in the photonic crystal core layer 3, soas to obtain a large sectioned area of the optical waveguide forobtaining a desired high optical coupling coefficient to the externaloptical system.

[0092] The propagation of the optical wave is not inhibited outside thephotonic band of the photonic crystal. In the vicinity of the photonicband edge of the photonic crystal, however, a dispersion is extremelylarge and a group velocity is extremely low, for which reason if awavelength of a propagating light is close to the photonic band edge ofthe photonic crystal, then this makes it possible to apply this novelphotonic crystal waveguide to various optical devices such as an opticaldispersion-compensating device, an optical pulse compression device andan optical delay circuit.

[0093] In this embodiment, the novel photonic crystal waveguide is madeof the Si/SiO₂ based materials over the silicon substrate. It is, ofcourse, possible that the novel photonic crystal waveguide is made ofother materials, for example, AlGaAs based materials over a GaAssubstrate, and InGaAsP based materials over an InP substrate.

[0094] In this embodiment, the top cladding layer comprises an airhaving a reflective index of 1. It is, of course, possible to furtherprovide a top cladding layer of the same material as the bottom claddinglayer to provide a three-dimensional photonic crystal layered structure.

[0095] Second Embodiment:

[0096] A second embodiment according to the present invention will bedescribed in detail with reference to the drawings. FIG. 3 is afragmentary perspective view illustrative of a second novel photoniccrystal waveguide in a second embodiment in accordance with the presentinvention.

[0097] In the above first embodiment, the resist pattern 4 is provided.This resist pattern 4, however, shows an optical absorption which casesa propagation loss. Notwithstanding, if the length of the opticalwaveguide in the optical propagation direction is not so long, then apropagation loss by the resist pattern 4 is not problem.

[0098] In this second embodiment, in place of the resist pattern 4, adielectric pattern 14 of other dielectric material than the resistmaterial is provided on the optical waveguide region, wherein thedielectric pattern 14 is lower in optical absorption coefficient thanthe resist material. The dielectric pattern 14 may be made of silicondioxide. Since the second novel photonic crystal waveguide uses thedielectric pattern 14 to suppress the optical absorption for reducingthe propagation loss. Even if the length of the optical waveguide in theoptical propagation direction is long, then the propagation loss by thedielectric pattern 14 is not problem. The second novel photonic crystalwaveguide is structurally different in the dielectric pattern 14 fromthe above described first novel photonic crystal waveguide.

[0099] The second novel photonic crystal waveguide has a two-dimensionalslab photonic crystal layered structure. This two-dimensional slabphotonic crystal layered structure comprises a bottom cladding layer 2over a silicon substrate 1 and a photonic crystal core layer 3 over thebottom cladding layer 2. The bottom cladding layer 2 is made of silicondioxide having a refractive index of about 1.5. A top surface of thephotonic crystal core layer 3 is exposed to an air. This air serves as atop cladding layer having a refractive index of 1. The photonic crystalcore layer 3 has arrays of through holes 30 which completely penetratethe photonic crystal core layer 3 and reach the top surface of thebottom cladding layer 2, wherein the through holes 30 are distributedentirely throughout the photonic crystal core layer 3 at a center pitchin the range of 0.6-0.8 micrometers. The diameter of the through holes30 may be about the wavelength of the propagating light. The centerpitch is defined to be a distance between centers of adjacent two of thedistributed through holes 30. The through holes 30 have a diameter ofabout 0.5 micrometers. The distribution of the through holes 30 maycomprise a two-dimensional periodical array, for example, in the form ofsquare-lattice, triangle-lattice or hexagonal-lattice.

[0100] In this case, an averaged refractive index of the photoniccrystal layer is approximately 2. In view of both the opticalconfinement and the coupling coefficient with the external opticalsystem, it is preferable that the silicon dioxide bottom cladding layer2 and the photonic crystal core layer 3 have a thickness of about 2micrometers. A dielectric layer is entirely formed over the top surfaceof the photonic crystal core layer 3, wherein the dielectric layer ismade of a dielectric material lower in optical absorption coefficientthan a resist material. A resist is further applied on the top surfaceof the dielectric layer. The resist is then patterned by lithographyprocesses to form a stripe-shaped resist mask over a waveguide region ofthe photonic crystal core layer 3. The stripe-shaped resist mask is usedfor a selective anisotropic etching to the dielectric layer to form astripe-shaped dielectric pattern 14 on the waveguide region of thephotonic crystal core layer 3. The used stripe-shaped resist mask isremoved. In the slab photonic crystal waveguide, the waveguide regioncovered by the stripe-shaped dielectric pattern 14 is higher inequivalent refractive index than the remaining part of the photoniccrystal core layer 3. A light is confined in the region which is higherin equivalent refractive index than the surrounding region of thephotonic crystal core layer 3, for which reason the light is confined inthe waveguide region which is higher in equivalent refractive index thanthe remaining part of the photonic crystal core layer 3. Accordingly,the waveguide region higher in equivalent refractive index serves as anoptical waveguide. The width of the stripe-shaped dielectric pattern 14defines a width of the optical waveguide. In view of both the opticalconfinement and the coupling coefficient with the external opticalsystem, it is preferable that the width of the stripe-shaped dielectricpattern 14 is about 2 micrometers, so that the width of the opticalwaveguide is about 2 micrometers. As described above, the preferablethickness of the photonic crystal core layer 3 is also 2 micrometers.Thus, the optical waveguide has a square-sectioned area of 2micrometers×2 micrometers, which is much larger than the aboveconventional line defect optical waveguide. This large sectioned area ofthe optical waveguide is suitable for obtaining a high optical couplingcoefficient to the external optical system.

[0101] In accordance with the second novel two-dimensionalslab-structured photonic crystal waveguide, in place of the conventionalline defect optical waveguide, the dielectric pattern 14 is provided fordefining the optical waveguide in the photonic crystal core layer 3, soas to obtain a large sectioned area of the optical waveguide forobtaining a desired high optical coupling coefficient to the externaloptical system.

[0102] In this second embodiment, in place of the resist pattern 4, adielectric pattern 14 of other dielectric material than the resistmaterial is provided on the optical waveguide region, wherein thedielectric pattern 14 is lower in optical absorption coefficient thanthe resist material. The dielectric pattern 14 may be made of silicondioxide. Since the second novel photonic crystal waveguide uses thedielectric pattern 14 to suppress the optical absorption for reducingthe propagation loss. Even if the length of the optical waveguide in theoptical propagation direction is long, then the propagation loss by thedielectric pattern 14 is not problem.

[0103] The propagation of the optical wave is not inhibited outside thephotonic band of the photonic crystal. In the vicinity of the photonicband edge of the photonic crystal, however, a dispersion is extremelylarge and a group velocity is extremely low, for which reason if awavelength of a propagating light is close to the photonic band edge ofthe photonic crystal, then this makes it possible to apply this novelphotonic crystal waveguide to various optical devices such as an opticaldispersion-compensating device, an optical pulse compression device andan optical delay circuit.

[0104] In this embodiment, the novel photonic crystal waveguide is madeof the Si/SiO₂ based materials over the silicon substrate. It is, ofcourse, possible that the novel photonic crystal waveguide is made ofother materials, for example, AlGaAs based materials over a GaAssubstrate, and InGaAsP based materials over an InP substrate.

[0105] In this embodiment, the top cladding layer comprises an airhaving a reflective index of 1. It is, of course, possible to furtherprovide a top cladding layer of the same material as the bottom claddinglayer to provide a three-dimensional photonic crystal layered structure.

[0106] Third Embodiment:

[0107] A third embodiment according to the present invention will bedescribed in detail with reference to the drawings. FIG. 4 is afragmentary perspective view illustrative of a third novel photoniccrystal waveguide in a third embodiment in accordance with the presentinvention. The third novel photonic crystal waveguide has a photoniccrystal layered structure with a ridged optical waveguide. This photoniccrystal layered structure comprises a bottom cladding layer 2 over asilicon substrate 1 and a photonic crystal core layer 3 over the bottomcladding layer 2. The bottom cladding layer 2 is made of silicon dioxidehaving a refractive index of about 1.5. A top surface of the photoniccrystal core layer 3 is exposed to an air. This air serves as a topcladding layer having a refractive index of 1. The photonic crystal corelayer 3 has a ridge-formed thicker portion 5 which is thicker than theremaining part of the photonic crystal core layer 3, wherein theridge-formed thicker portion 5 extends on the waveguide region. A lightis confined in the ridge-formed thicker portion 5 of the photoniccrystal core layer 3. The photonic crystal core layer 3 has arrays ofthrough holes 30 which completely penetrate the photonic crystal corelayer 3 and reach the top surface of the bottom cladding layer 2,wherein the through holes 30 are distributed entirely throughout thephotonic crystal core layer 3 at a center pitch in the range of 0.6-0.8micrometers, The diameter of the through holes 30 may be about thewavelength of the propagating light. The center pitch is defined to be adistance between centers of adjacent two of the distributed throughholes 30. The through holes 30 have a diameter of about 0.5 micrometers.The distribution of the through holes 30 may comprise a two-dimensionalperiodical array, for example, in the form of square-lattice,triangle-lattice or hexagonal-lattice.

[0108] In this case, an averaged refractive index of the photoniccrystal layer is approximately 2. In view of both the opticalconfinement and the coupling coefficient with the external opticalsystem, it is preferable that the silicon dioxide bottom cladding layer2 has a thickness of about 2 micrometers and the ridge-formed thickerportion 5 of the photonic crystal core layer 3 has a thickness of 2micrometers and the remaining portions of the photonic crystal corelayer 3 has a thickness of 1 micrometer.

[0109] Accordingly, the ridge-formed thicker portion 5 of the photoniccrystal core layer 3 serves as an optical waveguide. The width of theridge-formed thicker portion 5 defines a width of the optical waveguide.In view of both the optical confinement and the coupling coefficientwith the external optical system, it is preferable that the width of theridge-formed thicker portion 5 is about 2 micrometers, so that the widthof the optical waveguide is about 2 micrometers. As described above, thepreferable thickness of the ridge-formed thicker portion 5 of thephotonic crystal core layer 3 is also 2 micrometers. Thus, the opticalwaveguide has a square-sectioned area of 2 micrometers×2 micrometers,which is much larger than the above conventional line defect opticalwaveguide. This large sectioned area of the optical waveguide issuitable for obtaining a high optical coupling coefficient to theexternal optical system.

[0110] FIGS, 5A through 5C are fragmentary perspective viewsillustrative of the third novel photonic crystal waveguides insequential steps involved in the fabrication method therefor in thethird embodiment in accordance with the present invention.

[0111] With reference to FIG. 5A, a resist is applied on the top surfaceof the photonic crystal core layer 3. The resist is then patterned bylithography processes to form a stripe-shaped resist pattern 4 on thewaveguide region of the photonic crystal core layer 3, wherein thephotonic crystal core layer 3 has a uniform thickness, for example, of 2micrometers.

[0112] With reference to FIG. 5B, a dry etching process is then carriedout by use of the stripe-shaped resist pattern 4 as a mask forselectively etching the photonic crystal core layer 3, so that thethickness of the photonic crystal core layer 3 is reduced to 1micrometer, except under the stripe-shaped resist pattern 4. As aresult, the ridge-formed thicker portion 5 is formed under thestripe-shaped resist pattern 4.

[0113] With reference to FIG. 5C, the used stripe-shaped resist pattern4 is removed. The photonic crystal core layer 3 has the ridge-formedthicker portion 5 which has a thickness of 2 micrometers, whilst theremaining etched part of the photonic crystal core layer 3 has athickness of 1 micrometer.

[0114] The photonic crystal structure also extends around the opticalwaveguide comprising the ridge-formed thicker portion 5 of the photoniccrystal core layer 3. This photonic crystal structure around the opticalwaveguide suppress a leakage of an optical wave from the opticalwaveguide. This means that the third novel photonic crystal waveguide ishighly responsible to the requirement for abrupt vent or curve of theoptical path.

[0115] In accordance with the third novel photonic crystal waveguide,ridge-formed thicker portion 5 is provided for defining the opticalwaveguide in the photonic crystal core layer 3, so as to obtain a largesectioned area of the optical waveguide for obtaining a desired highoptical coupling coefficient to the external optical system.

[0116] The propagation of the optical wave is not inhibited outside thephotonic band of the photonic crystal. In the vicinity of the photonicband edge of the photonic crystal, however, a dispersion is extremelylarge and a group velocity is extremely low, for which reason if awavelength of a propagating light is close to the photonic band edge ofthe photonic crystal, then this makes it possible to apply this novelphotonic crystal waveguide to various optical devices such as an opticaldispersion-compensating device, an optical pulse compression device andan optical delay circuit.

[0117] In this embodiment, the novel photonic crystal waveguide is madeof the Si/SiO₂ based materials over the silicon substrate, It is, ofcourse, possible that the novel photonic crystal waveguide is made ofother materials, for example, AlGaAs based materials over a GaAssubstrate, and InGaAsP based materials over an InP substrate.

[0118] In this embodiment, the top cladding layer comprises an airhaving a reflective index of 1. It is, of course, possible to furtherprovide a top cladding layer of the same material as the bottom claddinglayer to provide a three-dimensional photonic crystal layered structure.

[0119] Fourth Embodiment:

[0120] A fourth embodiment according to the present invention will bedescribed in detail with reference to the drawings. FIG. 6 is afragmentary perspective view illustrative of a fourth novel photoniccrystal waveguide in a fourth embodiment in accordance with the presentinvention. The fourth novel photonic crystal waveguide has a photoniccrystal layered structure with a ridged optical waveguide. This photoniccrystal layered structure comprises a bottom cladding layer 2 over asilicon substrate 1 and a photonic crystal core layer 3 over the bottomcladding layer 2. The bottom cladding layer 2 is made of silicon dioxidehaving a refractive index of about 1.5. A top surface of the photoniccrystal core layer 3 is exposed to an air. This air serves as a topcladding layer having a refractive index of 1. The photonic crystal corelayer 3 has a ridge-formed thicker portion 5 which is defined by doublechannel regions, wherein the ridge-formed thicker portion 5 is thickerthan the double channel regions of the photonic crystal core layer 3.The ridge-formed thicker portion 5 extends on the waveguide region,whilst the double channel regions extend opposite longitudinal sides ofthe ridge-formed thicker portion 5. A light is confined in theridge-formed thicker portion 5 of the photonic crystal core layer 3. Thephotonic crystal core layer 3 has arrays of through holes 30 whichcompletely penetrate the photonic crystal core layer 3 and reach the topsurface of the bottom cladding layer 2, wherein the through holes 30 aredistributed entirely throughout the photonic crystal core layer 3 at acenter pitch in the range of 0.6-0.8 micrometers. The diameter of thethrough holes 30 may be about the wavelength of the propagating light.The center pitch is defined to be a distance between centers of adjacenttwo of the distributed through holes 30. The through holes 30 have adiameter of about 0.5 micrometers. The distribution of the through holes30 may comprise a two-dimensional periodical array, for example, in theform of square-lattice, triangle-lattice or hexagonal-lattice.

[0121] In this case, an averaged refractive index of the photoniccrystal layer is approximately 2. In view of both the opticalconfinement and the coupling coefficient with the external opticalsystem, it is preferable that the silicon dioxide bottom cladding layer2 has a thickness of about 2 micrometers and the ridge-formed thickerportion 5 of the photonic crystal core layer 3 has a thickness of 2micrometers and the double channel regions of the photonic crystal corelayer 3 has a thickness of 1 micrometer.

[0122] Accordingly, the ridge-formed thicker portion 5 of the photoniccrystal core layer 3 serves as an optical waveguide. The width of theridge-formed thicker portion 5 defines a width of the optical waveguide.In view of both the optical confinement and the coupling coefficientwith the external optical system, it is preferable that the width of theridge-formed thicker portion 5 is about 2 micrometers, so that the widthof the optical waveguide is about 2 micrometers. As described above, thepreferable thickness of the ridge-formed thicker portion 5 of thephotonic crystal core layer 3 is also 2 micrometers. Thus, the opticalwaveguide has a square-sectioned area of 2 micrometers×2 micrometers,which is much larger than the above conventional line defect opticalwaveguide. This large sectioned area of the optical waveguide issuitable for obtaining a high optical coupling coefficient to theexternal optical system.

[0123]FIGS. 7A through 7C are fragmentary perspective views illustrativeof the fourth novel photonic crystal waveguides in sequential stepsinvolved in the fabrication method therefor in the fourth embodiment inaccordance with the present invention.

[0124] With reference to FIG. 7A, the bottom cladding layer 2 is formedover the silicon substrate 1. The photonic crystal core layer 3 isformed over the bottom cladding layer 2, wherein the photonic crystalcore layer 3 has a uniform thickness, for example, of 2 micrometers.

[0125] With reference to FIG. 7B, a focused ion beam is irradiated onselected double channel regions of the photonic crystal core layer 3 forselectively etching the photonic crystal core layer 3, so that thethickness of the double channel regions of the photonic crystal corelayer 3 is reduced to 1 micrometer.

[0126] With reference to FIG. 7C, the ridge-formed thicker portion S isformed which is defined between the thickness-reduced double channelregions of the photonic crystal core layer 3. The photonic crystal corelayer 3 has the ridge-formed thicker portion 5 which has a thickness of2 micrometers, whilst the double channel regions of the photonic crystalcore layer 3 have a thickness of 1 micrometer.

[0127] The photonic crystal structure also extends around the opticalwaveguide comprising the ridge-formed thicker portion 5 of the photoniccrystal core layer 3. This photonic crystal structure around the opticalwaveguide suppress a leakage of an optical wave from the opticalwaveguide. This means that the fourth novel photonic crystal waveguideis highly responsible to the requirement for abrupt vent or curve of theoptical path. The etched area of the photonic crystal core layer 3 issmaller than what is described in the above third embodiment. The focusion beam process makes it unnecessary to carry out the lithographyprocess made in the above third embodiment.

[0128] In accordance with the fourth novel photonic crystal waveguide,ridge-formed thicker portion S is provided for defining the opticalwaveguide in the photonic crystal core layer 3, so as to obtain a largesectioned area of the optical waveguide for obtaining a desired highoptical coupling coefficient to the external optical system.

[0129] The propagation of the optical wave is not inhibited outside thephotonic band of the photonic crystal. In the vicinity of the photonicband edge of the photonic crystal, however, a dispersion is extremelylarge and a group velocity is extremely low, for which reason if awavelength of a propagating light is close to the photonic band edge ofthe photonic crystal, then this makes it possible to apply this novelphotonic crystal waveguide to various optical devices such as an opticaldispersion-compensating device, an optical pulse compression device andan optical delay circuit.

[0130] In this embodiment, the novel photonic crystal waveguide is madeof the Si/SiO₂ based materials over the silicon substrate. It is, ofcourse, possible that the novel photonic crystal waveguide is made ofother materials, for example, AlGaAs based materials over a GaAssubstrate, and InGaAsP based materials over an InP substrate.

[0131] Fifth Embodiment:

[0132] A fifth embodiment according to the present invention will bedescribed in detail with reference to the drawings. FIG. 8 is afragmentary perspective view illustrative of a fifth novel photoniccrystal waveguide to be used as a directional coupler-in a fifthembodiment in accordance with the present invention. FIG. 9 is afragmentary perspective view illustrative of the fifth novel photoniccrystal waveguide of FIG. 8 in use. The fifth novel photonic crystalwaveguide has a photonic crystal layered structure with a double-ridgedoptical waveguide structure. This photonic crystal layered structurecomprises a bottom cladding layer 2 over a silicon substrate 1 and aphotonic crystal core layer 3 over the bottom cladding layer 2. Thebottom cladding layer 2 is made of silicon dioxide having a refractiveindex of about 1.5. A top surface of the photonic crystal core layer 3is exposed to an air. This air serves as a top cladding layer having arefractive index of 1. The photonic crystal core layer 3 has doubleridge-formed thicker portions 15 and 5, for example, a main ridge-formedthicker portion 5 and a subordinate ridge-formed thicker portion 15, Themain and subordinate ridge-formed thicker portions 15 and 5 extend inparallel to each other and are distanced at a constant pitch The mainand subordinate ridge-formed thicker portions 15 and 5 are thicker thanthe remaining portions of the photonic crystal core layer 3. The mainand subordinate ridge-formed thicker portions 15 and 5 extend on themain and subordinate waveguide regions. A light is confined in eitherone of the main and subordinate waveguide regions comprising the mainand subordinate ridge-formed thicker portions 15 and 5 of the photoniccrystal core layer 3. A velocity of the light in either one of the mainand subordinate waveguide regions comprising the main and subordinateridge-formed thicker portions 15 and 5 of the photonic crystal corelayer 3 is much smaller than that of the conventional directionalcoupler with the ridged waveguides made of other material than thephotonic crystal, This allows a remarkable size reduction of the noveldirectional coupler using the photonic crystal. A light may be incidentinto one of the main and subordinate waveguide regions 15 and 5. A partof the incident light is introduced into another of the main andsubordinate waveguide regions 15 and 5, whilst the remaining part of theincident light remains propagated through the incident one of the mainand subordinate waveguide regions 15 and 5. The photonic crystal corelayer 3 has arrays of through holes 30 which completely penetrate thephotonic crystal core layer 3 and reach the top surface of the bottomcladding layer 2, wherein the through holes 30 are distributed entirelythroughout the photonic crystal core layer 3 at a center pitch in therange of 0.6-0.8 micrometers. The diameter of the through holes 30 maybe about the wavelength of the propagating light. The center pitch isdefined to be a distance between centers of adjacent two of thedistributed through holes 30. The through holes 30 have a diameter ofabout 0.5 micrometers. The distribution of the through holes 30 maycomprise a two-dimensional periodical array, for example, in the form ofsquare-lattice, triangle-lattice or hexagonal-lattice.

[0133] In this case, an averaged refractive index of the photoniccrystal layer is approximately 2. In view of both the opticalconfinement and the coupling coefficient with the external opticalsystem, it is preferable that the silicon dioxide bottom cladding layer2 has a thickness of about 2 micrometers and the main and subordinateridge-formed thicker portions 15 and 5 of the photonic crystal corelayer 3 have a thickness of 2 micrometers and the remaining region ofthe photonic crystal core layer 3 has a thickness of 1 micrometer.

[0134] Accordingly, the main and subordinate ridge-formed thickerportions 15 and 5 of the photonic crystal core layer 3 serve as the mainand subordinate optical waveguides. The widths of the main andsubordinate ridge-formed thicker portions 15 and 5 defines widths of themain and subordinate optical waveguides. In view of both the opticalconfinement and the coupling coefficient with the external opticalsystem, it is preferable that the widths of the main and subordinateridge-formed thicker portions 15 and 5 are about 2 micrometers, so thatthe widths of the main and subordinate optical waveguides are about 2micrometers As described above, the preferable thickness of the main andsubordinate ridge-formed thicker portions 15 and 5 of the photoniccrystal core layer 3 is also 2 micrometers. Thus, each of the main andsubordinate optical waveguides has a square-sectioned area of 2micrometers×2 micrometers, which is much larger than the aboveconventional line defect optical waveguide. This large sectioned area ofthe optical waveguide is suitable for obtaining a high optical couplingcoefficient to the external optical system.

[0135] Accordingly the use of the photonic crystal for the directionalcoupler allows the remarkable size reduction of the directional coupler.

[0136] In this embodiment, the novel photonic crystal waveguide is madeof the Si/SiO₂ based materials over the silicon substrate. It is, ofcourse, possible that the novel photonic crystal waveguide is made ofother materials, for example, AlGaAs based materials over a GaAssubstrate, and InGaAsP based materials over an InP substrate.

[0137] In this embodiment, the top cladding layer comprises an airhaving a reflective index of 1. It is, of course, possible to furtherprovide a top cladding layer of the same material as the bottom claddinglayer to provide a three-dimensional photonic crystal layered structure.

[0138] Sixth Embodiment:

[0139] A sixth embodiment according to the present invention will bedescribed in detail with reference to the drawings. FIG. 10 is afragmentary perspective view illustrative of a sixth novel photoniccrystal waveguide in a sixth embodiment in accordance with the presentinvention. FIG. 11 is a fragmentary perspective view illustrative of aseventh novel photonic crystal waveguide in this sixth embodiment inaccordance with the present invention. FIGS. 12A and 12B are fragmentaryperspective views illustrative of a novel method of forming the photoniccrystal waveguide in this sixth embodiment in accordance with thepresent invention. FIG. 13 is a fragmentary perspective viewillustrative of an eighth novel photonic crystal waveguide in this sixthembodiment in accordance with the present invention.

[0140] The sixth novel photonic crystal waveguide has a two-dimensionalslab photonic crystal layered structure. This two-dimensional slabphotonic crystal layered structure comprises a bottom cladding layer 2having a thickness of 2 micrometers over a silicon substrate 1 and aphotonic crystal core layer 3 having a thickness of 2 micrometers overthe bottom cladding layer 2. The bottom cladding layer 2 is made ofsilicon dioxide having a refractive index of about 1.5. A top surface ofthe photonic crystal core layer 3 is exposed to an air. This air servesas a top cladding layer having a refractive index of 1. The photoniccrystal core layer 3 may be made of silicon. The photonic crystal corelayer 3 has arrays of through holes 30 which completely penetrate thephotonic crystal core layer 3 and reach the top surface of the bottomcladding layer 2, wherein the through holes 30 are distributed entirelythroughout the photonic crystal core layer 3 at a center pitch in therange of 0.6-0.8 micrometers. The diameter of the through holes 30 maybe about the wavelength of the propagating light. The center pitch isdefined to be a distance between centers of adjacent two of thedistributed through holes 30. The through holes 30 have a diameter ofabout 0.5 micrometers. The distribution of the through holes 30 maycomprise a two-dimensional periodical array, for example, in the form ofsquare-lattice, triangle-lattice or hexagonal-lattice. The sixth novelphotonic crystal waveguide has an I-shaped waveguide 5, whilst theseventh photonic crystal waveguide has an Y-shaped waveguide 5. Thethrough holes 30 except on the waveguide region 5 are filled with theair having the reflective index of 1. The through holes 30 on thewaveguide region 5 are filled with a material having a refractive indexof higher than 1. The reflective index of the waveguide region 5 ishigher than the remaining part of the photonic crystal core layer 3, sothat the light is confined in the waveguide region 5. The availablematerials having the higher refractive indexes than 1 are, for example,dielectric materials such as silicon dioxide and a resist material. Ifthe resist material is used as a filler into the through holes on thewaveguide region, then the following method may be available.

[0141] With reference to FIG. 12A, a resist 7 is applied on an entiretop surface of the photonic crystal core layer 3. A photo-mask 8 is usedwhich has a waveguide pattern for carrying out an exposure process.

[0142] With reference to FIG. 12B, a development is carried out todissolve the exposed resist, wherein the unexposed resist remains on thewaveguide region, whereby the waveguide is formed in the photoniccrystal core layer 3.

[0143] The preferable resist materials are preferably small inabsorption loss in the long wavelength band. One of the preferableresist materials is, for example, poly-methyl-methacrylate (PMMA).Polyimide and BCB are also available as the filler materials in thethrough holes in place of the resist material because polyimide and BCBare low in absorption loss in the long wavelength band.

[0144] The eighth novel photonic crystal waveguide uses those materialsother than the resist as shown in FIG. 13. The filler material has atemperature coefficient having an inverse sign to the temperaturecoefficient of the photonic crystal of the core layer 3 for cancelingthe temperature variation and realizing athermalization or thetemperature-independency.

[0145] In this embodiment, the novel photonic crystal waveguide is madeof the Si/SiO₂ based materials over the silicon substrate. It is, ofcourse, possible that the novel photonic crystal waveguide is made ofother materials, for example, AlGaAs based materials over a GaAssubstrate, and InGaAsP based materials over an InP substrate.

[0146] In this embodiment, the top cladding layer comprises an airhaving a reflective index of 1. It is, of course, possible to furtherprovide a top cladding layer of the same material as the bottom claddinglayer to provide a three-dimensional photonic crystal layered structure.

[0147] As further modifications to the foregoing embodiments, it ispossible to optically combine two or three of the first or secondembodiment, the third or fourth embodiment, and the sixth embodiment. Itis also possible to apply the above waveguide structure to variousoptical devices such as directional copular.

[0148] Whereas modifications of the present invention will be apparentto a person having ordinary skill in the art, to which the inventionpertains, it is to be understood that embodiments as shown and describedby way of illustrations are by no means intended to be considered in alimiting sense. Accordingly, it is to be intended to cover by claims allmodifications which fall within the spirit and scope of the presentinvention.

What is claimed is:
 1. A photonic crystal waveguide comprising: asubstrate; a bottom cladding layer over said substrate; and a core layerover said bottom cladding layer, said core layer having a uniformdistribution of holes; wherein said core layer has at least a waveguideregion which is thicker than a remaining region of said core layer tocause a refractive index guide effect.
 2. The photonic crystal waveguideas claimed in claim 1 , wherein said waveguide region has a ridgedshape.
 3. The photonic crystal waveguide as claimed in claim 1 , whereina plurality of said waveguide region extends in parallel to each otherand distanced from each other to form a directional copular.
 4. Thephotonic crystal waveguide as claimed in claim 1 , wherein said corelayer is made of such a photonic crystal material that a wavelength of alight to be propagated through said waveguide region is in the vicinityof a photonic band gap edge of said photonic crystal material in orderto utilize an intense dispersion phenomenon.
 5. The photonic crystalwaveguide as claimed in claim 1 , wherein said uniform distribution ofsaid holes comprises a two-dimensional periodical array of through holesat a constant pitch between centers of adjacent two of said throughholes.
 6. The photonic crystal waveguide as claimed in claim 1 , whereinsaid uniform distribution of said holes comprises a three-dimensionalperiodical array of holes at a constant pitch between centers ofadjacent two of said through holes.
 7. The photonic crystal waveguide asclaimed in claim 1 , wherein said holes are filled with an air.
 8. Thephotonic crystal waveguide as claimed in claim 1 , further comprising atop cladding layer over said core layer.
 9. The photonic crystalwaveguide as claimed in claim 1 , wherein a dielectric pattern isprovided on said waveguide region, and said dielectric pattern has arefractive index higher than a substance in contact with a top surfaceof said core layer.
 10. The photonic crystal waveguide as claimed inclaim 1 , wherein said holes except on said waveguide region are filledwith an air, whilst said holes on said waveguide regions are filled witha filler material having a refractive index higher than
 1. 11. Thephotonic crystal waveguide as claimed in claim 10 , wherein a dielectricpattern is provided on said waveguide region, and said dielectricpattern has a refractive index higher than a substance in contact with atop surface of said core layer.
 12. A photonic crystal waveguidecomprising: a substrate; a bottom cladding layer over said substrate;and a core layer with a uniform thickness over said bottom claddinglayer, said core layer having a uniform distribution of holes; whereinsaid core layer has at least a waveguide region, on which a dielectricpattern is provided which has a refractive index higher than a substancein contact with a top surface of said core layer.
 13. The photoniccrystal waveguide as claimed in claim 12 , wherein said substance incontact with said top surface of said core layer is an air, and saidrefractive index of said dielectric pattern is higher than
 1. 14. Thephotonic crystal waveguide as claimed in claim 12 , further comprising atop cladding layer over said core layer, and said top cladding layer ismade of the same material as said bottom cladding layer, and saidsubstance in contact with said top surface of said core layer is thesame material as said bottom cladding layer, and said refractive indexof said dielectric pattern is higher than said top cladding layer. 15.The photonic crystal waveguide as claimed in claim 12 , wherein aplurality of said waveguide region extends in parallel to each other anddistanced from each other to form a directional copular.
 16. Thephotonic crystal waveguide as claimed in claim 12 , wherein said corelayer is made of such a photonic crystal material that a wavelength of alight to be propagated through said waveguide region is in the vicinityof a photonic band gap edge of said photonic crystal material in orderto utilize an intense dispersion phenomenon.
 17. The photonic crystalwaveguide as claimed in claim 12 , wherein said uniform distribution ofsaid holes comprises a two-dimensional periodical array of through holesat a constant pitch between centers of adjacent two of said throughholes.
 18. The photonic crystal waveguide as claimed in claim 12 ,wherein said uniform distribution of said holes comprises athree-dimensional periodical array of holes at a constant pitch betweencenters of adjacent two of said through holes.
 19. The photonic crystalwaveguide as claimed in claim 12 , wherein said holes are filled with anair.
 20. The photonic crystal waveguide as claimed in claim 12 , whereinsaid waveguide region is thicker than a remaining region of said corelayer to cause a refractive index guide effect.
 21. The photonic crystalwaveguide as claimed in claim 12 , wherein said holes except on saidwaveguide region are filled with an air, whilst said holes on saidwaveguide regions are filled with a filler material having a refractiveindex higher than
 1. 22. The photonic crystal waveguide as claimed inclaim 21 , wherein said waveguide region is thicker than a remainingregion of said core layer to cause a refractive index guide effect. 23.A photonic crystal waveguide comprising: a substrate; a bottom claddinglayer over said substrate; and a core layer with a uniform thicknessover said bottom cladding layer, said core layer having a uniformdistribution of holes; wherein said core layer has at least a waveguideregion, and said holes except on said waveguide region are filled withan air, whilst said holes on said waveguide regions are filled with afiller material having a refractive index higher than
 1. 24. Thephotonic crystal waveguide as claimed in claim 23 , wherein a pluralityof said waveguide region extends in parallel to each other and distancedfrom each other to form a directional copular.
 25. The photonic crystalwaveguide as claimed in claim 23 , wherein said core layer is made ofsuch a photonic crystal material that a wavelength of a light to bepropagated through said waveguide region is in the vicinity of aphotonic band gap edge of said photonic crystal material in order toutilize an intense dispersion phenomenon.
 26. The photonic crystalwaveguide as claimed in claim 23 , wherein said uniform distribution ofsaid holes comprises a two-dimensional periodical array of through holesat a constant pitch between centers of adjacent two of said throughholes.
 27. The photonic crystal waveguide as claimed in claim 23 ,wherein said uniform distribution of said holes comprises athree-dimensional periodical array of holes at a constant pitch betweencenters of adjacent two of said through holes.
 28. The photonic crystalwaveguide as claimed in claim 23 , further comprising a top claddinglayer over said core layer.
 29. The photonic crystal waveguide asclaimed in claim 23 , wherein said filler material has a temperaturecoefficient which is inverse in sign to a temperature coefficient of abase material of said core layer.
 30. The photonic crystal waveguide asclaimed in claim 23 , wherein said waveguide region is thicker than aremaining region of said core layer to cause a refractive index guideeffect.
 31. The photonic crystal waveguide as claimed in claim 23 ,wherein a dielectric pattern is provided on said waveguide region, andsaid dielectric pattern has a refractive index higher than a substancein contact with a top surface of said core layer.
 32. The photoniccrystal waveguide as claimed in claim 31 , wherein said waveguide regionis thicker than a remaining region of said core layer to cause arefractive index guide effect.
 33. A directional coupler comprising: asubstrate; a bottom cladding layer over said substrate; and a photoniccrystal core layer over said bottom cladding layer, said core layerhaving a uniform distribution of holes, and said core layer being madeof such a photonic crystal material that a wavelength of a light to bepropagated through said waveguide region is in the vicinity of aphotonic band gap edge of said photonic crystal material in order toutilize an intense dispersion phenomenon; and said core layer having apair of stripe-shaped waveguide regions which extends in parallel toeach other, wherein said stripe-shaped waveguide regions are thickerthan a remaining region of said core layer to cause a refractive indexguide effect.
 34. The directional coupler as claimed in claim 33 ,wherein said uniform distribution of said holes comprises atwo-dimensional periodical array of through holes at a constant pitchbetween centers of adjacent two of said through holes.
 35. Thedirectional coupler as claimed in claim 33 , wherein said uniformdistribution of said holes comprises a three-dimensional periodicalarray of holes at a constant pitch between centers of adjacent two ofsaid through holes.
 36. The directional coupler as claimed in claim 33 ,further comprising a top cladding layer over said core layer.
 37. Thedirectional coupler as claimed in claim 33 , wherein said waveguideregion has a ridged shape.
 38. The directional coupler as claimed inclaim 1 , wherein said holes are filled with an air.
 39. A directionalcoupler comprising: a substrate; a bottom cladding layer over saidsubstrate; and a photonic crystal core layer over said bottom claddinglayer, said core layer having a uniform distribution of holes, and saidcore layer being made of such a photonic crystal material that awavelength of a light to be propagated through said waveguide region isin the vicinity of a photonic band gap edge of said photonic crystalmaterial in order to utilize an intense dispersion phenomenon; and saidcore layer having a pair of stripe-shaped waveguide regions whichextends in parallel to each other, wherein dielectric patterns isprovided on said waveguide regions, and said dielectric patterns have arefractive index higher than a substance in contact with a top surfaceof said core layer.
 40. The directional coupler as claimed in claim 39 ,wherein said substance in contact with said top surface of said corelayer is an air, and said refractive index of said dielectric pattern ishigher than
 1. 41. The directional coupler as claimed in claim 39 ,further comprising a top cladding layer over said core layer, and saidtop cladding layer is made of the same material as said bottom claddinglayer, and said substance in contact with said top surface of said corelayer is the same material as said bottom cladding layer, and saidrefractive index of said dielectric pattern is higher than said topcladding layer.
 42. The directional coupler as claimed in claim 39 ,wherein said uniform distribution of said holes comprises atwo-dimensional periodical array of through holes at a constant pitchbetween centers of adjacent two of said through holes.
 43. Thedirectional coupler as claimed in claim 39 , wherein said uniformdistribution of said holes comprises a three-dimensional periodicalarray of holes at a constant pitch between centers of adjacent two ofsaid through holes.
 44. The directional coupler as claimed in claim 12 ,wherein said holes are filled with an air.
 45. A directional couplercomprising: a substrate; a bottom cladding layer over said substrate;and a photonic crystal core layer over said bottom cladding layer, saidcore layer having a uniform distribution of holes, and said core layerbeing made of such a photonic crystal material that a wavelength of alight to be propagated through said waveguide region is in the vicinityof a photonic band gap edge of said photonic crystal material in orderto utilize an intense dispersion phenomenon; and said core layer havinga pair of stripe-shaped waveguide regions which extends in parallel toeach other, wherein said core layer has at least a waveguide region, andsaid holes except on said waveguide region are filled with an air,whilst said holes on said waveguide regions are filled with a fillermaterial having a refractive index higher than
 1. 46. The directionalcoupler as claimed in claim 45 , wherein said uniform distribution ofsaid holes comprises a two-dimensional periodical array of through holesat a constant pitch between centers of adjacent two of said throughholes.
 47. The directional coupler as claimed in claim 45 , wherein saiduniform distribution of said holes comprises a three-dimensionalperiodical array of holes at a constant pitch between centers ofadjacent two of said through holes.
 48. The directional coupler asclaimed in claim 45 , further comprising a top cladding layer over saidcore layer.
 49. The directional coupler as claimed in claim 45 , whereinsaid filler material has a temperature coefficient which is inverse insign to a temperature coefficient of a base material of said core layer.